The present invention relates to a technique for cutting a crystal ingot with wire cutting apparatus, and in particular to such a technique utilizing wire with a diameter smaller than 0.18 mm, such that less of the crystal ingot is wasted as the wire cuts through the crystal ingot.
In various industries, particularly the electronic industry, thin wafers of material are utilized in the fabrication of the components. The wafers are typically sliced from a larger body of the material known as a crystal ingot. For instance, large bodies of polycrystalline or monocrystalline silica, GaAs, InP, GGG (gadolinium-gallium garnet), quartz, synthetic sapphire, and/or ceramic materials may be sliced into wafers, which are used to fabricate various components.
As a specific example, the semiconductor industry typically manufactures semiconductor substrates that are used to fabricate integrated circuits from single crystal ingots. These ingots are commonly grown by a standard melt crystal growth technique, such as the Czochralski (CZ) method. In the CZ method, a generally cylindrical single crystal is pulled vertically from a silicon melt in a heated crucible. The growth is initiated by dipping a small seed crystal in the melt, and after the thermal equilibrium is reached, the crystal is pulled upward so that it grows with a relatively constant diameter. At the same time, the crystal ingot and the crucible are rotated in opposite directions. This process results in a single crystal ingot that has a generally constant radius, uniform dopant and impurity distribution, low number of defects, and continuous CZ growth.
Once the single crystal ingot is formed, further processing is necessary to shape the cylindrical structure so as to have a prescribed diameter. This processing typically involves centering the single ingot crystal in a shaping device, such as a lathe, and then grinding the ingot to the prescribed diameter dimension. Once the ingot has been properly ground to the required diameter, the ingot is then sliced perpendicular to the longitudinal axis to obtain generally planar wafers. The resulting wafers are then further processed by lapping, etching, and polishing both the major surface(s) and the edge, prior to forming integrated circuits or other semiconductor devices thereupon.
An ingot is typically sliced into wafers with a wire cutting apparatus. A wire cutting apparatus includes a thin wire arranged to have a number of parallel wire segments. By advancing the wire in a reciprocating fashion, and pushing the ingot through the parallel wire segments, the ingot is cut into wafers. The spacing of the wires correlates to the desired thickness of the resulting wafers. The wires are usually disposed on cylindrical rollers, which define grooves to receive the wire. The rollers are spaced apart to create the parallel wire segments through which the ingot is advanced.
In order to advance the wire, at least one of the rollers is typically rotationally driven. In one conventional embodiment of the wire cutting apparatus, three rollers are arranged in a substantially triangular configuration, such as the wire cutting apparatus commercially available from Nippei Toyama Company of Tokyo, Japan. In another conventional embodiment of the wire cutting apparatus, four rollers are arranged in a substantially rectangular configuration, such as the wire cutting apparatus commercially available from HCT Shaping Systems SA of Cheseaux, Switzerland.
To facilitate cutting the crystal ingot with the wires, a slurry is applied to the wire prior to the wire engaging the crystal ingot. The slurry is typically a mixture of an abrasive material and a coolant. When multiple wire segments span across the rollers to cut the crystal ingot into multiple wafers, the slurry is applied to the wire segments and generally forms a relatively continuous sheet between the wire portions. As such, the wire carries at least some of the abrasive as it passes through the crystal ingot. The sheet of slurry must be uniform across the wire segments for the wire to slice the crystal ingot uniformly. When the sheet of slurry is not applied uniformly, the wires cut the ingot differently, thereby producing wafers that disadvantageously exhibit warp. Warp is generally defined as the maximum deviation of a wafer from a best fit plane through the wafer. Although a small amount of warp in wafers may be acceptable for some applications, excessive warp generally renders a wafer unusable. Thus, it is desirable to minimize warp in a wafer by ensuring that the slurry is evenly distributed on and carried by the wire segments.
The slurry typically is applied to the wire segments via nozzles that spray the slurry toward the wire. The nozzles may be any shape or type of dispenser capable of spraying the desired amount and desired type of slurry onto the wire. For example, a nozzle may be a bar or a rod that extends across the wire segments and defines openings through which the slurry sprays. As such, the nozzle is oriented relative to the wire segments so that the slurry sprays onto the wire segments. For most wire cutting apparatus, particularly wire cutting apparatus with three rollers, two nozzles are positioned on each side of the location where the crystal ingot advances through the wire segments. Thus, two nozzles may be positioned proximate to each roller located on each side of the crystal ingot. The nozzles typically collectively disperse the slurry at a rate of 80 liters/minute. The viscosity of the slurry is typically 71 centipose, and the temperature of the slurry is generally between 26 and 28 degrees Celsius.
The wire utilized to slice crystal ingots typically has a diameter of 0.18 mm or more. As the wire cuts into the crystal ingot, however, a portion of the crystal ingot is destroyed by the wire, which is known as kerf loss. For example, a wire with a 0.18 mm diameter creates a slit having a width of at least 0.18 mm between adjacent wafers as the wires move through the crystal ingot. Since many wafers are cut from a single ingot, the cumulative kerf loss can quickly become significant.
Therefore, there is a desire in the industry for a technique for cutting crystal ingots with wire having a smaller diameter so as to decrease the kerf loss during the cutting process without requiring significant and/or expensive changes to existing wire cutting apparatus. Further, the technique should produce wafers that exhibit minimal, if any, warp or other type of undesirable properties. Unfortunately, some attempts to utilize wire having a diameter less than 0.18 mm have resulted in wafers having unacceptably large warp.